Bug fix - added auth_args method to MySQL::Diff::Database.

RT 79976: bin/mysqldiff was calling MySQL::Diff::Database->auth_args,
which did not exist. The stated workaround in the RT bug DB was to use
_auth_args_string directly. The solution was to just create a wrapper
method called auth_args. Added a unit test for regression checking.

Bug URL - https://rt.cpan.org/Public/Bug/Display.html?id=79976